I swear by the eBags brand Mother Lode eTech duffels. We have 2 of the "Junior" variety, which are the mid-sized ones. It's extremely well made, designed, and durable. My 2 bags have been through baggage handling 16 times so far, and they still look brand new. Even the dirt from baggage handling wipes off easily with a damp cloth, since they're made of ballistic nylon. eBags has a lifetime warranty as well. If you wait around for a 20% off sale (which is pretty often if you google for a coupon), the price isn't too bad. These are called duffels, but they're not floppy like a traditional duffel.
I used to buy cheaper luggage from
Walmart, Target, etc but it kept getting destroyed after 1 or 2 trips. Samsonite and American Tourister quality is spotty now, depending on which model you buy. Even my Delsey suitcase (which was expensive!) only lasted 2 trips before the frame was shattered. I paid around $115-$120 each for these eBags duffels, but I seriously think they'll last me forever.
They also have a larger version of the bag, but you'd have to be careful not to go over the 50 lb limit since they're so big, and you can put more in them. The medium size version works well for 1 week trips.
These are the ones I have:
http://www.ebags.com/ebags/mother_lode_etech_junior_25_wheeled_duffel/product_detail/index.cfm?modelid=54633 There's another version of it called TLS, which is almost double the price, mostly because it is about 2 lbs lighter and is expandable. I have the eTech version, which is less expensive.